Field of the Invention
The present invention relates to position detecting devices that detect the relative position between a head and a scale, such as a magnetic scale or an optical scale.
Description of the Related Art
Conventionally, as the position detecting device that detects precise displacement position, such as linear displacement or rotational displacement, there is known a position detecting device including a magnetic scale and a magnetism detecting element. The position detecting device is widely utilized, for example, in the electronic-component mounting apparatuses that need accurate positioning control of a conveyed article, the detection (measurement) devices that detect (measure) the dimensions of a component, and the like.
FIG. 1 illustrates the fundamental configuration of a conventional magnetic-type position detecting device. The position detecting device in FIG. 1 detects linear displacement, and includes a magnetic scale 1 including a magnetic medium. In the magnetic scale 1, the magnetization directions of the S pole and N pole are reversed for each constant distance. One unit, in which the S pole and N pole are repeated, is one wavelength of the recording signal of the magnetic scale 1.
Then, in the position detecting device of FIG. 1, a magnetic head 5 is prepared that moves along the longitudinal direction (x direction in the view), in which the magnetization direction of the magnetic scale 1 is reversed, and two magnetic sensors 5a and 5b are arranged in the magnetic head 5. The respective magnetic sensors 5a and 5b are the sensors that detect the magnetism leaking into a space from the magnetization recorded in the near magnetic scale 1, and are arranged spaced apart from each other by the distance corresponding to ¼ period of one wavelength (1×λ) of a reproduced signal. Therefore, one magnetic sensor 5a detects the recording signal of the magnetic scale 1 as a sinusoidal signal (SIN signal), while the other magnetic sensor 5b detects the recording signal of the magnetic scale 1 as a cosine signal (COS signal).
Here, the relative position between the magnetic scale 1 and the magnetic head 5 is represented by x. The relative position x is calculated from the SIN signal sin(x) output by the magnetic sensor 5a and the COS signal cos(x) output by the magnetic sensor 5b. Note that sin(x)/cos(x) is calculated using the SIN signal and COS signal to obtain tan(x), and then from the value of the tan(x), the relative position x between the magnetic scale 1 and the magnetic head 5 within one wavelength is obtained (interpolation), so that the position x is calculated with a resolution finer than the length of one wavelength. The information about the calculated position is transmitted to a positional information display device and control apparatus.
FIG. 2 illustrates an arrangement example of the magnetic scale and magnetism detecting elements of a conventional magnetic-type position detecting device. The example of FIG. 2 illustrates a case of detecting linear displacement, and includes the magnetic scale 1 including a magnetic medium. In the magnetic scale 1, the magnetization directions of the S pole and N pole are reversed for each constant distance. One unit, in which the S pole and N pole are repeated, is one wavelength of the recording signal of the magnetic scale 1.
Then, the position detecting device includes a detecting section 2 having magnetism detecting elements 3a to 3h arranged at positions near the magnetic scale 1. As the magnetism detecting elements 3a to 3h, AMR (Anisotropic Magneto-Resistance) elements are used which utilize the anisotropy resistance effect, for example. In the magnetism detecting device, the magnetic scale 1 is arranged on the fixed side and the detecting section 2 is arranged on the movable side, so that the position detecting device detects the relative position between the magnetic scale 1 and the detecting section 2.
FIG. 3 illustrates an arrangement example of eight magnetism detecting elements 3a to 3h. The upper side of FIG. 3 is an element arrangement seen from the top face of the magnetic scale 1, while the lower side of FIG. 3 is an element arrangement seen in a cross sectional direction of the magnetic scale 1.
The magnetic scale 1 is magnetized into the N pole and S pole at a predetermined interval in the longitudinal direction. Then, as the magnetic signal detected by the magnetism detecting element, one period during which the N pole and S pole alternately change is one wavelength λ. A half of one wavelength λ is one pitch P. The N pole and S pole are linearly arranged at one pitch interval.
Then, close to the magnetic scale 1, four magnetism detecting elements 3a to 3d are arranged close to each other. As the arrangement interval of these four magnetism detecting elements 3a to 3d, as illustrated in FIG. 3, two magnetism detecting elements 3a and 3b are arranged at the interval of one pitch P, and the other two magnetism detecting elements 3c and 3d are arranged at the interval of one pitch P. Then, the magnetism detecting elements 3a and 3c are spaced apart from each other by (n+1/2)p. Here, n is an integer. These four magnetism detecting elements 3a to 3d are connected in series. A series circuit having these four magnetism detecting elements 3a to 3d connected in series is connected between a portion at a predetermined potential V and a ground potential portion GND, and a signal Ch+ is extracted from the middle point (i.e., connection point between the magnetism detecting elements 3b and 3c) of the series circuit.
Furthermore, other four magnetism detecting elements 3e to 3h are arranged spaced apart by a predetermined distance (m+1/2)P from these four magnetism detecting elements 3a to 3d. Here, m is an integer. The arrangement interval of these four magnetism detecting elements 3e to 3h is the same as that of the magnetism detecting elements 3a to 3d, and these four magnetism detecting elements 3e to 3h are connected in series. A series circuit having these four magnetism detecting elements 3e to 3h connected in series is connected between the portion at the predetermined potential V and the ground potential portion GND, and a signal Ch− is extracted from the middle point (i.e., connection point between the magnetism detecting elements 3f and 3g) of the series circuit.
Note that the magnetism detecting elements 3a to 3h each have a P/6-step portion roughly at the center thereof. The P/6-step portion is for removing the 3rd order distortion from the detection signal.
FIG. 4 illustrates a connection configuration for obtaining a detection signal from these eight magnetism detecting elements 3a to 3h. 
The signal Ch+ obtained from the middle point of four magnetism detecting elements 3a to 3d and the signal Ch− obtained from the middle point of four magnetism detecting elements 3e to 3h are supplied to an operational amplifier 4. In the operational amplifier 4, both the signals Ch+ and Ch− are differentially amplified and extracted as the detection signal.
By extracting the signal detected by the magnetism detecting elements 3a to 3h with the configurations illustrated in FIG. 3 and FIG. 4, a detection signal whose distortion has been cancelled is obtained. That is, because among the four magnetism detecting elements 3a to 3d, the elements 3a and 3b and the elements 3c and 3d are arranged at the interval of ¼ of one wavelength of the recording signal, the signal changes detected by the respective sets of the elements 3a and 3b and of the elements 3c and 3d have opposite phases. Then, these four magnetism detecting elements 3a to 3d are connected in series and the signal Ch+ is extracted from the middle point of the series circuit, so that the even order distortions of the detection signal are canceled.
Another set of four magnetism detecting elements 3e to 3h are similarly connected and the signal Ch− is extracted, so that the even order distortions of the detection signal are canceled.
Furthermore, these signals Ch+ and Ch− are supplied to the operational amplifier 4 so that a detection signal amplified by the operational amplifier 4 is obtained.
The configurations illustrated in FIG. 3 and FIG. 4 are for obtaining one detection signal, and therefore in obtaining a plurality of signals from the magnetic scale 1, the same number of the detecting sections 2 illustrated in FIG. 3 as the number of the detection signals are arranged. For example, when the recording signal of the magnetic scale 1 needs to be detected as the sinusoidal signal (SIN signal) and the cosine signal (COS signal), two sets of the detecting sections 2 illustrated in FIG. 3 are needed. When the −SIN signal and the −COS signal are obtained, further two sets of the detecting sections 2 are needed. Patent Literature 1 describes an example of the magnetic-type position detecting device.